In the United States, religion is usually said to be a personal affair – one’s own private decision about how and what to believe. But this remarkably private and individualistic approach is somewhat odd when compared with the vast majority of cultures and religions throughout history. Far more often, religion has been a public affiliation, determining cultural identities, affecting marriage and family choices, and defining groups in relation to each other. A fascinating recent study published in PLOS Genetics shows just how inextricable religion often is from culture, finding that religious identity has decisively shaped the genetic landscape of the Levant – so decisively, in fact, that Lebanese Muslims are more closely related to fellow Muslims from Morocco or Yemen than they are to their Christian or Jewish compatriots.